If you want to save power and reduce costs, you need to know how much power each appliance uses over a period of time. Most appliances don’t run all the time, so you need to know the power they use while they are actually running and how much they use over the longer term.

The unit is easy to build, with all parts mounted on two PC boards. Pt.2 next month has the assembly details.

The easiest way to determine that is to use an electronic power meter and this new "Energy Meter" fits the bill nicely. It displays the measured power in Watts, the elapsed time and the total energy usage in kWh. In addition, it can show the energy cost in dollars and cents. As a bonus, it also includes comprehensive brownout protection.

One obvious use for this unit it to show refrigerator running costs over a set period of time, so that you can quickly determine the effect of different thermostat settings. Alternatively, it could be used to show the difference in energy consumption between the summer months and the winter months.

If you have a solar power installation, this unit will prove invaluable. It will quickly allow you to determine which appliances are the most "power hungry", so that you can adjust your energy usage patterns to suit the capacity of the installation. And there are lots of other uses – for example, the unit could be used to determine the cost of pumping water, the running costs of an aquarium or even the cost of keeping your TV set on standby power, so that it can be switched on via the remote control.